nilfs2: remove unlikely directive causing mis-conversion of error code

The following error code handling in nilfs_segctor_write() function
wrongly converted negative error codes to a truth value (i.e. 1):

   err = unlikely(err) ? : res;

which originaly meant to be

   err = err ? : res;

This mis-conversion caused that write or sync functions receive the
unexpected error code.  This fixes the bug by removing the unlikely
directive.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: stable@kernel.org